With age, buried steel transmission pipelines transporting pressurized fluid can develop defects in the pipe wall. The defect may be a corrosion pit that extends only part way through the wall and thus is still non-leaking. Or it may be an aperture extending through the wall, in which case there is an actual leak.
Up until recently, when either leaking or non-leaking defects were discovered it was the common practice to empty the line, excavate the line at the defect and encapsulate the defective span in what amounted to a containment pressure vessel welded to the line. More particularly, a pair of semi-cylindrical flanged shells, having an internal peripheral protruding seal or gasket, would be bolted onto the line to provide a full encirclement sleeve. Circumferential fillet welds would then be applied between the sleeve ends and the pipeline wall, to effect completion of the pressure vessel. A typical example of such a sleeve is illustrated in U.S. Pat. No. 4,535,822, issued to Rogers.
A recent development in the pipeline repair art is disclosed in Canadian Patent 1,237,682, issued to Stata and Robinson and assigned to the present applicant. This patent teaches a full encirclement reinforcement sleeve system, for non-leaking defects. The system involves creating a thick-walled cylinder at the defect. This is done by clamping semi-circular shells against the pipeline wall with known force and then welding the shells longitudinally to yield a thick-walled cylinder having uniform hoop stress.
More particularly, in accordance with this system:
A pair of semi-cylindrical shells having lapped ends are welded together at their first ends and then the free second ends are pulled together, using upstanding lugs secured to the free ends and a clamping device pulling on the lugs, to cause the shells to clamp sufficiently tightly and evenly to the line to effectively form a thick-walled cylinder;
The clamping means is hydraulically actuated. This makes it possible to monitor the amount of clamping force being applied, so that a measure of the increasing extent of hoop stress being induced in the sleeve can be determined;
The clamping means are further designed to leave the free lapped ends of the two shells accessible for longitudinal welding; and
The sleeve is welded longitudinally along the shell ends, to produce a reinforcing thick-walled cylinder.
However, the Stata and Robinson prior art system has limitations. These limitations include:
That the system is not adapted to seal a defect that is actually leaking, since welding of the sleeve ends cannot be carried out in the vicinity of an actual leak; and
That the system involves using lugs, upstanding from the shell ends, that are pulled on by the hydraulic clamping means. These lugs preclude installing an outer reinforcement sleeve over the thick-walled cylinder, if this is desired.
Still another pertinent prior art concept and means to be considered is disclosed in U.S. Pat. No. 2,520,802, issued to Hampton. This patent teaches a pipe patch system involving:
an annular domed seal element that is positioned against a leaking containment wall, to encircle the leak;
a steel shell that has an inwardly opening recess, so that the seal element is partly contained by the recess wall;
and use of an outer clamp means, comprising a pair of semi-circular flanged shells bolted together at the flanges, for pressing against the shell to compress the seal element against the containment wall around the leak.
Another prior art patent of interest, relating to use of annular seal elements in the context of pipeline patches, is U.S. Pat. No. 1,737,181, issued to Woodward.
In still another prior art teaching of interest, U.S. Pat. No. 2,616,736, issued to T. L. Smith, discloses forming a gasket with a ribbed surface on one side. The raised ribs are provided in a cross-hatched pattern. The concept involved is that if fluid leaks past one rib, it will be contained by the next.